Hey, everyone. So in this example question, it says, a solvating agent which has a molar mass of 147 grams per mole that contains carbon, hydrogen, and chlorine is used for spectroscopic processes. Now, determine its molecular formula when a 0.250-gram sample creates 0.451 grams of carbon dioxide and 0.0617 grams of water upon combustion. To solve this question, we're going to employ the following steps. And step 1 is, if present, convert the grams of carbon dioxide to grams of just carbon. Alright. So we're given 0.451 grams of carbon dioxide. The mass of carbon dioxide, one carbon and two oxygens, is 44.01 grams. Again, remember, that comes from adding up one carbon plus two oxygens. We find that from the periodic table for their atomic masses. And that's equal to one mole of carbon dioxide. We're gonna say here, one mole of carbon dioxide has within it, one mole of just carbon. And according to the periodic table, one mole of carbon weighs 12.01 grams. When we work this out, we're gonna get 0.1231 grams of carbon. To avoid rounding errors, make sure you at least use 4 decimal places when doing these types of calculations. Here, I'm giving this as our value for now. Next, convert the grams of water into grams of hydrogen here. So we're gonna have here 0.0617 grams of water. Water is composed of two hydrogens and one oxygen. The combined mass from the periodic table is 18.016 grams, and that's for every one mole of water. Now from the formula, we see that water has in it two hydrogens. So we're gonna have two moles of hydrogen for every one mole of water, and one mole of hydrogen according to the periodic table weighs 1.008 grams. This gives me 0.0069 grams of hydrogen. Now, step 3, if necessary, subtract the grams of steps 1 and 2 from the grams of the sample to determine the third element. So, here we have 0.250 grams of our sample which is made up of carbon, hydrogen, and chlorine. We're gonna subtract out the masses that we just found for carbon and hydrogen. Which would mean that the difference would be the grams of chlorine, which comes out to be 0.1200 grams of chlorine. So we have the grams of all three of our elements. So step 4 says, convert all those masses into moles. Right. So here, we're gonna basically just change each of these into moles by dividing them by their atomic masses. So carbon, one mole of carbon is 12.01 grams. One mole of hydrogen is 1.008 grams. One mole of chlorine, according to the periodic table, is 35.45 grams. All the grams cancel out and we'll have moles of each, which I'm going to write down here. So here for carbon, we'd have 0.01025 moles of carbon. We'd have 0.00685 moles of hydrogen, and we have 0.00339 moles of chlorine. Step 5, it's a long process. Step 5, divide each mole answer by the smallest mole value in order to obtain whole numbers for each element. So the smallest mole answer that we got was 0.00339. So all of them get divided by that number. Now step 6, if you get after you divide them all by the smallest moles, if you get a value of some number point 1 or some number, point 9, then you can round to the nearest whole number. If you can't round, we multiply by a factor to create whole numbers. Luckily, when we divide all these by their lowest mole number, we get whole numbers. We get three carbons here, we get two hydrogens here, and we get one chlorine here. This will give me the empirical formula which would be C3H2Cl. But remember, they're not asking us for the empirical formula, they're asking for the molecular formula. To figure out the molecular formula, we need to figure out what our n value will be. Your n value will equal your molecular mass or molar mass, which was given to us in the very beginning, divided by the empirical mass. So we're told in the very beginning that it weighs 147 grams per mole. We have to find the empirical mass. The empirical mass comes from the empirical formula. We have three carbons, two hydrogens, one chlorine. Multiply them all by their atomic masses from the periodic table. Right? So that'd be 36.03 grams, 2.016 grams, and 35.45 grams. When you add those all up together, we get 73.496 grams per mole. Again, this 73 comes from adding up these numbers in your calculator. So when we do that, we get n equals 2. So now we're going to say that our molecular formula, we're gonna take this number 2 here, and multiply it by the empirical formula. When we do that, we'll get our molecular formula, which comes out to be C 6 H 4 Cl2. So this would be our final answer. That would represent our molecular formula.
Table of contents
- 1. Intro to General Chemistry3h 46m
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- 23. Chemistry of the Nonmetals2h 39m
- Main Group Elements: Bonding Types4m
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- Periodic Table Charges Review20m
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- Production of Hydrogen8m
- Group 1A and 2A Reactions7m
- Boron Family Reactions7m
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- Nitrogen Family Reactions12m
- Oxides, Peroxides, and Superoxides12m
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- Noble Gas Compounds3m
- 24. Transition Metals and Coordination Compounds3h 16m
- Atomic Radius & Density of Transition Metals11m
- Electron Configurations of Transition Metals7m
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- Ligands10m
- Complex Ions5m
- Coordination Complexes7m
- Classification of Ligands11m
- Coordination Numbers & Geometry9m
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- Isomerism in Coordination Complexes14m
- Orientations of D Orbitals4m
- Intro to Crystal Field Theory10m
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- Magnetic Properties of Complex Ions9m
- Strong-Field vs Weak-Field Ligands6m
- Magnetic Properties of Complex Ions: Octahedral Complexes11m
3. Chemical Reactions
Combustion Analysis
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